Energy storing by means of a fly wheel is utilized today at excenter presses and has been proposed for large hydraulic presses (DE-A1 No. 1.627.843). In such hydraulic driven presses a fly wheel is arranged between the electric motor and the pump for the hydraulic fluid in such a way that the electric motor which is necessary for the operation may be constructed relatively small whereby an essential saving of power may be achieved. The pump must be dimensioned with respect to the highest pressure which may occur and for that reason may be comparatively large but since the pressing work is executed by the fly wheel the electric motor needs only be dimensioned for the movement of the piston. By this design, a power saving is achieved which in itself is valuable, but there is no energy saving because a small part (a few percent) of the energy provided is converted to useful work and the larger part is consequently lost.
The same applies for such driving devices for presses which use pressure accumulators for storing energy at a small total effect. An example of such a device is disclosed in No. SE-8-7513069-0, where the energy stored in the pressure accumulator is completely utilized, as it is not necessary to use a throttling device for converting the existing pressure in the accumulator. There is, however, no recovery of energy provided with this device.
A common measure in hydraulic plants is to arrange the quick transport of the piston in a press by means of an accumulator. During the return stroke the force is only a few percent of the total pressing force at which the accumulator is loaded. This plant only admits an insignificant storing of energy and no recovery of energy at all.
The object of the invention is to provide an essential energy saving, i.e. recovering of energy, at the same time as the previous advantageously power saving is maintained or even is further improved. This has been achieved whereby at least one driving unit connectable to the hydraulic system in the machine is arranged to bring back the elastically stored energy in the machine to the energy accumulator (7) during the decompression phase of the machine.
Through the invention the following advantages are achieved: the energy saving is obtained while the hydraulic pump during the decompression phase is working as a hydraulic motor, whereby an energy recover of between 60-90% is obtained. The power saving is of the same size and is obtained by that the connnecting power for the electric motor or motors may be decreased, which results in considerable savings in the costs for installation and connection. Further important savings of costs may be obtained for cooling of the hydraulic oil if the water for the cooling must be bought. Dependent on the water costs this saving may be up to three times higher than the saving on the electric side.